Techniques are conventionally known in which fibers are mixed with rubber used for rubber goods, such as tires, to improve properties such as hardness and modulus. In such techniques, fibers with a large fiber diameter tend to disperse well in rubber but reduce rubber physical properties such as fatigue resistance, whereas fibers with a small fiber diameter tend to improve the fatigue resistance but be entangled with each other and disperse poorly in rubber.
Against this, there is proposed mixed yarn fibers having a sea-island cross-section, which are dispersed in rubber and become fibrillated by a shearing force at mixing to increase the area of contact with rubber, whereby both dispersibility and fatigue resistance can be achieved (see Patent Document 1). These fibers, however, form a sea-island structure due to phase separation of resin and therefore have ununiform thicknesses and lengths, and have diameters as large as 1 μm and 0.7 μm as described in an example, meaning that the size of the area of contact with rubber is less than sufficient; therefore, a great reinforcing effect cannot be expected.
Examples of Patent Document 2 disclose that for improved wear resistance, addition of bacterial cellulose having a very small fiber diameter of 0.1 μm together with starch that serves as a reinforcing agent to diene rubber improves the wear resistance index as compared to addition of starch alone. In Patent Document 2, however, it is described that addition of cellulose alone has a problem with processability, and starch is added in an amount five times or more the amount of cellulose. It is considered that the starch is added in an attempt to improve the dispersibility because bacterial cellulose is dispersed to nano sizes in water but tends to aggregate in rubber, but in this case, it is expected that the reinforcing effect is balanced out by the starch, and the reinforcing effect is still not sufficient.
Patent Document 3 discloses examples in which impalpable powder cellulose fibers with an average particle size of 40 μm are introduced in a dry state into a rubber composition together with a silane coupling agent, and kneaded in a Banbury mixer. However, just by kneading in a mixer, it is difficult to break hydrogen bonds between the cellulose fibers caused in a dry state to make the cellulose fibers into fibers with a small diameter, and in this case, the cellulose fibers are considered to be dispersed in the rubber still in the form of particles with an average particle size of 40 μm. Therefore, the reinforcing effect of thin long fibers cannot be expected.
Furthermore, Patent Document 4 discloses a vulcanized rubber composition containing a rubber component comprising at least one of natural rubber, modified natural rubber, and synthetic rubber, and chemically modified microfibril cellulose, and discloses examples in which modified microfibril cellulose with an average fiber diameter of 0.1 μm is mixed with the rubber component. The examples disclose stirring microfibril cellulose in advance in water using a rotary homogenizer to prepare a dispersion, introducing rubber latex thereinto, and mixing the resultant at 7000 rpm for 10 minutes. In this case, although a rotary homogenizer is used, the fibers tend to aggregate before water is removed, and a shearing force sufficient to disentangle the aggregated fibers is not produced at such a rotation speed. In the document, the absence of an aggregate is visually confirmed, but actually, it is not clear that at what thickness the microfibril cellulose is dispersed in the rubber.
Furthermore, Patent Document 5 proposes using as cellulose fibers obtained by graft polymerization of a diene polymer to increase the affinity and dispersibility in a rubber component. In this case, however, fibers defibrated in water are subjected to graft treatment in tetrahydrofuran (THF), and at this treatment, the fibers defibrated once in water are considered to reaggregate. If strong intermolecular hydrogen bonds are formed once, it is difficult to defibrate the fibers to nano sizes again.